(1) Field of the Invention
The present invention relates to a capstan motor for use in a video tape recorder, digital audio tape recorder, cassette tape recorder, etc.
(2) Description of the Prior Art
A conventional tape driving system for VTRs etc., needs to incorporate torque limiter mechanisms each having different limits, respectively for supply and takeup side reel tables. Therefore, the reel tables become complicated as to their structure and hence high in cost as well as becoming greater in weight and in height at the reel table portions. In order to solve these problems, methods of incorporating a limiter mechanism into a capstan motor have been disclosed (in, for example, Japanese Utility Model Application Laid-Open Hei 5 No. 96,921 and Japanese Patent Application Laid-Open Hei 5 No. 182,299).
A conventional example will be explained with reference to FIG. 1. In FIG. 1, a capstan shaft 1 is rotatably held in a housing 2 by a bearing 3. This housing 2 is fixed to a stator yoke 4. Fixed to stator yoke 4 on the side opposite to housing 2 is a coil 5. A boss 8 is press fitted on the lower portion of capstan shaft 1. This boss 8 has a rotor yoke 7 fixed thereto, on which a magnet 6 is attached opposing coil 5. A limiter magnet 53 is fixed on the bottom side of rotor yoke 7. A hysteresis plate 50 is rotatably attached relative to capstan shaft 1 so as to oppose the limiter magnet. This hysteresis plate has an output wheel 51 which is provided around the circumference of hysteresis plate 50 and allows mechanical power to be extracted.
According to this capstan motor, when rotor yoke 7 rotates, limiter magnet 53 also rotates, which produces a hysteresis torque in hysteresis plate 50, hence the plate 50 also rotates. This rotational force of hysteresis plate 50 is picked up through output wheel 51 for reel driving. This limiter portion can absorb the difference between the rotating speed of the reel which changes with the variation in the wound diameter of the tape on the reel and the rotational speed of the capstan motor which rotates at a fixed rate. Therefore, it is no longer necessary to provide any torque limiter in the takeup reel table.
Another conventional example will be described referring to FIG. 2. Description will be made of only parts which are different from the conventional example of FIG. 1, and the same or similar parts as in FIG. 1 will be allotted with the same reference numerals with no description. A coil fixing member 61 is used to attach a coil 5 to a stator yoke 4. A rotor yoke 7 is fixed to a capstan shaft 1 via a boss 8 so that a magnet 6 and a hysteresis plate 60 oppose each other with coil 5 in between. Hysteresis plate 60 is attached rotatably with respect to capstan shaft 1. A capstan gear 62 is provided around the circumference of the plate 60 so as to allow driving force to be extracted. As rotator yoke 7 rotates, this produces a hysteresis torque in hysteresis plate 60, hence capstan gear 62 also rotates. This rotational force of capstan gear 62 is used for reel driving. The rotating speed of the reel changes with the variation in the wound diameter of the tape on the reel while the capstan motor rotates at a fixed rate. The limiter portion can absorb this difference.
However, the above conventional configurations have the following problems. The method of FIG. 1, disclosed in Japanese Utility Model Application Laid-Open Hei 5 No. 96,921, needs an extra magnet 53 for limiting torque, in rotor yoke 7 of the capstan motor. This complicates the shape of rotor yoke 7 resulting in increased cost. Further, when a compact capstan motor is attempted, magnet 6 needs to be smaller than coil 5 because of the presence of torque limiter magnet 53; this may affect the motor characteristics.
Next, in the method of FIG. 2, disclosed in Japanese Patent Application Laid-Open Hei 5 No. 182,299, when no limiter torque occurs, rotor yoke 7 and hysteresis plate 60 integrally rotate. On the other hand, when the motor is used with limiting torque generated, hysteresis plate 60 rotates differently to rotor yoke 7, so that the magnetic flux passing through coil 5 varies to thereby affect the motor characteristics. Further, due to the magnetic characteristic unique to hysteresis plate 60, the motor characteristics will degrade. Moreover, since coil 5 is fixed to stator yoke 4 by means of coil fixing member 61, assembly of coil 5 needs to be done by controlling the central position of the coil and its gap with respect to the rotor yoke 7, needing a complicated arrangement with high precision.